1. Field of the Invention
The present invention relates to a process for remedying a soil contaminated with a pollutant and an apparatus used therefor.
2. Related Background Art
A large amount of chemical compounds or chemical products have been produced with the recent rapid progress in technology and science. Many of these are slowly accumulated in the environment and contaminate the environment. In particular, land where humans live is most easily affected by artificial contamination, and since the environmental water is circulating among the land, hydrosphere, and atmosphere, the environmental pollution of the land is a serious problem which might be expanded to the global level. Well-known examples of soil (land) contaminants include organochloric compounds such as trichloroethylene (TCE), tetrachloroethylene (PCE) and dioxin, aromatic compounds such as toluene, xylene and benzene, and fuel such as gasoline. Organochloric compounds such as trichloroethylene and tetrachloroethylene were once extensively used for washing precision parts as well as for dry cleaning, and contamination of soil and underground water on a large scale due to the leakage of the organochloric compounds are now being revealed. Since such organochloric chemicals are teratogenic and carcinogenic and adversely affect the biosphere, purification of the polluted soil or ground water is now an issue to be solved immediately in addition to the isolation of the pollution source.
Methods for remedying soil polluted with these pollutants include, for example, heat treatment of the dug-up soil, vacuum extraction of the contaminant from the polluted soil and microbial degradation of the pollutant in soil. Although the heat treatment can completely purify the soil, the soil must be dug up since soil under structures cannot otherwise be purified. Moreover, this method is unsuitable for large-scale treatment because of the immense costs of digging and heating. In addition, the organochlorine compounds heated and evaporated from the soil which cause air pollution must be recovered by adsorption to activated carbon and the like, and then the used activated carbon requires safe disposal.
On the other hand, vacuum extraction and bioremediation do not require digging up soil, therefore they are inexpensive and simple processes and can be applied to the soil under the buildings while the ground surface is occupied by the buildings. The vacuum extraction process, however, requires the same treatment of the recovered organochlorine compounds as with the heat treatment method. In Japanese Patent Application Laid-Open No. 7-185252, compact ground equipment for vacuum extraction was proposed, utilizing a fluidized bed for regeneration of activated carbon to which the abstracted gas has been adsorbed. Ground equipment is still required for the treatment.
For the bioremediation, there are two methods, one utilizes the pollutant degrading microorganisms originally living in the soil, so-called native microorganisms, and the other utilizes pollutant degrading microorganisms not originally living in the soil. By the former method, remediation is carried out by introducing into the soil activating agents such as nutrients, inducers, oxygen and growth stimulating agents to improve the degradation activity of the native microorganisms.
The examples of strains capable of degrading TCE are given as follows: Welchia alkenophila sero 5 (U.S. Pat. No. 4,877,736, ATCC 53570), Welchia alkenophila sero 33 (U.S. Pat. No. 4,877,736, ATCC 53571), Methylocystis sp. strain M (Agric. Biol. Chem., 53, 2903 (1989), Biosci. Biotech. Biochem., 56, 486 (1992), ibid. 56, 736 (1992)), Methylosinus trichosporium OB3b (Am. Chem. Soc. Natl. Meet. Dev. Environ. Microbiol., 29, 365 (1989), Appl. Environ. Microbiol., 55, 3155 (1989), Appl. Biochem. Biotechnol., 28, 877 (1991), Japanese Patent Application Laid-Open No. (JPUPA) 02-92274, JPUPA 03-292970), Methylomonas sp. MM2 (Appl. Environ. Microbiol., 57, 236 (1991)), Alcaligenes denitrificans ssp. xylosoxidans JE75 (Arch. microbiol., 154, 410 (1990)), Alcaliqenes eutrophus JMP134 (Appl. Environ. Microbiol., 56, 1179 (1990)), Mycobacterium vaccae JOB5 (J. Gen. Microbiol., 82, 163 (1974), Appl. Environ. Microbiol., 54, 2960 (1989), ATCC 29678), Pseudomonas putida BH (Journal of Japan Sewage Work Assosiation, 24, 27 (1987)), Pseudomonas sp. strain G4 (Appl. Environ. Microbiol., 52, 383 (1986), ibid. 53, 949 (1987), ibid. 54, 951 (1988), ibid. 56, 1279 (1990), ibid. 57, 1935 (1991), U.S. Pat. No. 4,925,802, ATCC 53617, this strain was originally classified as Pseudomonas cepacia then classified into Pseudomonas sp.), Pseudomonas mendocina KR-1 (Bio/Technol., 7, 282 (1989)), Pseudomonas putida F1 (Appl. Environ. Microbiol., 54, 1703 (1988), ibid. 54, 2578 (1988)), Pseudomonas fluorescens PFL12 (Appl. Environ. Microbiol., 54, 2578 (1988)), Pseudomonas putida KWI-9 (JPUPA 06-70753), Pseudomonas cepacia KKO1 (JPUPA 06-227769), Pseudomonas sp. (JPUPA 02-273599), Nitrosomonas europaea (Appl. Environ. Microbiol., 56, 1169 (1990)), Lactobacillus vaginalis sp. nov (Int. J. Syst. Bacteriol., 39, 368 (1989), ATCC 49540) and so on. All these strains, however, require a chemical substance such as aromatic compounds and methane as an inducer in order to express their TCE degrading activity.
According to the latter method, remediation is carried out by introducing foreign microorganisms into the soil together with microbial activator for enhancing degradation activity. It is economically desired that microorganisms or chemical substances in an amount as small as possible are introduced and spread into the region to be remedied to carry out the degradation of the pollutants for remediation of the soil. In this sense, the microbial treatment has a drawback since it requires the introduction of a huge amount of a treating solution to remedy a wide region, filling the pore space of the soil to be treated.
With both methods, it is difficult to confine the injected microorganisms or activating agents within a certain region and also difficult to recover the degrading bacteria once proliferated in the soil or the activating agents remaining in the soil after the treatment, causing problems of secondary contamination of the soil.
As mentioned above, the vacuum extraction process and bioremediation process have advantages over the heat treatment method, yet they still have some problems to be solved for more efficient treatment.
In Japanese Patent Application Laid-Open No. 6-254537 and No. 7-112176, it is proposed to combine the vacuum extraction process and the bioremediation process so that air and ground water in the soil contaminated with organochlorine compounds are exposed to vacuum and introduced to a bioreactor on the ground in which the pollutant is degraded. This method aims to solve the drawback of the vacuum extraction process, that is, the reprocessing of the recovered organochlorine compounds, by carrying out biodegradation of the organochlorine compounds, and to solve the drawback of the microbial process such as the injection of the treating solution and the secondary contamination by installing a microbial degradation reactor.
The above-mentioned process in which the vacuum extraction process and the microbial process are combined, still requires ground equipment including a biodegradation reactor, and in this sense, it has the same problem as that of the vacuum extraction process. Since the degradation activity of the microorganism depends on the temperature, an apparatus or equipment to keep the bioreactor at a constant temperature or to protect it from the ambient temperature is further required to assure certain decomposition efficiency. Thus, there is a problem that the scale of the ground equipment of this method becomes larger than that of the vacuum extraction alone.
In other words, large-scale equipment may increase the remediation cost, and huge ground equipment may cancel out the advantage of the vacuum extraction process and the bioremediation process over the heat treatment method, that is, the remediation can be carried out maintaining the state of the soil and the ground surface.